1. Field of the Invention
The present invention relates to a set of toners, a set of development agents, a process cartridge, and an image forming apparatus.
2. Description of the Background Art
In electrophotography, electrostatic recording, electrostatic printing, etc., latent electrostatic images are formed on an image bearing member (photoreceptor, photoconductor); the latent electrostatic image is developed with a development agent containing toner to obtain a visible image (toner image); and the visible image is transferred to a recording medium followed by fixing to obtain a fixed image.
Development agents are classified into a single component development agent formed of magnetic toner or non-magnetic toner and a two component development agent formed of toner and a carrier.
With regard to fixing, a heat roller system in which a heating roller is directly pressed against the toner image on the recording medium is widely used. However, the heat roller system requires a large amount of electric power for fixing.
To save energy, various approaches have been studied including a sleep mode to reduce the consumption power of the heating roller. For example, the power output of the heater for heating roller is reduced while images are not produced and increased while images are output to raise the temperature of the heating roller.
However, users are obliged to wait for some several tens of seconds for the system to return from the sleep mode before the heating roller is ready again for fixing. In addition, it is desirable to completely power off the heater to cut the power consumption.
To meet these demands, it is necessary to reduce the fixing temperature of the toner itself, and lower the fixing temperature of the toner while it is actually used.
With regard to the toner for use in a development agent, with advances in electrophotographic technology, toner having excellent low-temperature fixing ability and preservability (blocking resistance) has come to be sought. Accordingly, polyester resins have been tried because these exhibit superior low-temperature fixing ability and a higher affinity with recording media than do polystyrene resins, which typically have been used as binder resins for toner.
For example, Japanese Patent Application Publication No. JP-2004-245854-A describes an approach using toner containing a linear polyester resin stipulating specific physical properties such as molecular weight. JP-H4-70765-A describes an approach using toner containing a non-linear cross-linking type polyester resin using rosins as the acid component.
Binder resins available in the market are still inadequate to satisfy current demand with regard to the performance speed of an image forming apparatus and the reduction of the energy consumption thereof. It is extremely difficult to maintain the fixing strength if the fixing time is set to be shorter in the fixing process and the heating temperature of the fixing device is set to be low.
Toner containing polyester resins using rosins as described in JP-H4-70765-A mentioned above have excellent low-temperature fixing ability and an advantage that toner productivity by the pulverization method is improved because the toner has excellent pulverization property. Furthermore, by using 1,2-propane diol, which is a branch-type alcohol having three carbon atoms as the alcohol component, low-temperature fixing ability is improved without degrading offset resistance compared to an alcohol having one or two carbon atoms, and deterioration of preservability due to a drop in glass transition temperature is avoided when compared with a branch-type alcohol having four or more carbon atoms. Using such a polyester resin as the binder resin for toner makes it possible to conduct fixing at a low temperature and improve preservability.
However, although low-temperature fixing ability continues to improve by using a polyester having excellent low-temperature fixing ability, it is difficult in the near future to satisfy ever-greater demand for energy efficiency simply by using polyester resin alone.
JP-2006-208609-A describes introducing a fixing helping component into the toner to improve low-temperature fixing ability, thereby creating toner having a good combination of high-temperature preservation property and low-temperature fixing ability by making the fixing helping component present in the toner as crystal domains. JP-2009-109971-A and JP2006-337872-A describe using toner having a good combination of high-temperature preservation property and low-temperature fixing ability by introducing a crystalline polyester resin into the toner. However, as apparatus performance improves, toner is required to satisfy demand for both high durability and further energy efficiency simultaneously, which is difficult to do.
On the other hand, with regard to the releasing agent, JP-H8-278662-A, JP-H8-334920-A, JP-H10-161347-A, and JP-2000-321815-A describe manufacturing toner having excellent low-temperature fixing ability, hot offset resistance, and blocking resistance by adding releasing agents to the toner. However, particularly when these toners are used in a high-speed image forming apparatus, the combination of hot offset resistance and low-temperature fixing ability of the toners is not so good, and even when they have good low-temperature offset resistance and low-temperature fixing ability at the same time, the toners are slightly inferior in blocking resistance, resulting in deterioration of development property or an inability to maintain good offset resistance at both low temperatures and high temperatures.
JP-2004-246345-A describes an image forming apparatus providing excellent preservability for an extended period of time by controlling the dispersion of the releasing agent to ameliorate blocking resistance in addition to improving offset resistance and fixing ability.
However, when images having a high image area ratio are processed in the image forming apparatus employing this system for a long run length, a minute amount of the wax (releasing agent) contained in the toner to release the toner during fixing remains on the fixing member when releasing the toner. This inevitably happens to obtain the releasing property while preventing occurrence of offset. However, since this minute amount of wax remaining on the fixing member is in a high-temperature state, the wax volatilizes, attaches to, and accumulates on or around the fixing device. Thereafter, the attached and accumulated wax flows in a block due to radiation heat and causes production of defective images on which oil is attached.
JP-H08-44110-A describes a toner having a volatile component in an amount of less than 0.1% by weight and a wax component having a maximum peak in a temperature range of from 70° C. to 130° C. along the differential scanning calorimetry (DSC) curve during a temperature rise, with the maximum heat generation peak during a temperature descent in the range of around +9° C. to −9° C. relative to the maximum peak temperature. This toner has improved fixing ability and hot offset resistance so that the obtained toner has no adverse impact on the photoreceptor and the development agent bearing member.
However, the phenomenon that isolated wax volatilizes and accumulates in the fixing portion, resulting in contamination in the image forming apparatus, stems from the volatility of the wax itself. Therefore, although successful in some degree, the cause of the contamination still remains, so that production of abnormal images on which oil is attached is not completely prevented even if the content of the volatile component in the toner is regulated and reduced.
In addition, with regard to color printers and photocopiers employing electrophotography which have come to be widely used in recent years, although reproduction of full color images thereby is relatively good, the full color machines are slow in forming monochrome images relative to printers and photocopiers dedicated to produce monochrome images. In addition, monochrome images produced by the full color machines are glossy relative to those produced by the monochrome machines. Therefore, the full color machines need improving in some cases.
In particular, since low gloss (matte) monochrome images are popular, glossy monochrome images produced by such color image forming apparatuses are not selected due to their gloss. Therefore, with regard to black toner, the same resin as contained in the black toner for use in dedicated machines and/or a resin having a relatively high softening point are used for the color machines to reduce the gloss.
For example, JP-H6-148935-A describes a method of controlling the gloss by regulating the molecular weight distribution and the melting viscosity of the resin component of black toner and changing the amount of heat applied when fixing monochrome images and when fixing color images. However, even when the properties of the black toner are regulated, the gloss of color toners and the black toner may become noticeable in color reproduction. In such a case, photo images containing black portions such as human faces look visually undesirable due to the gloss difference. In particular, color images containing both letters and photos have a large gloss difference between the highlight portion and the letter portions, resulting in production of visually undesirable images.
In addition, even when the color and black portions have similar gloss in color reproduction, the development amounts for color photo images are different with regard to color photo images, for example, less with the pale color portion but more with the black portion in particular. In such a color image, gloss tends to be not uniform over the image, resulting in production of a visually undesirable image in which the black portion is extremely glossy.
To solve this problem, JP-H10-268562-A describes an approach of regulating the gloss difference between color toner portions and black toner portions in color images. However, the black toner still remains glossy when producing monochrome images. Therefore, it is difficult for color machines producing monochrome images using this approach to gain acceptance.